1. Field of the Invention
The present invention relates to an information display. Particularly, the present invention relates to a self light emitting display, further, to an electroluminescence (EL) display. Moreover, the present invention relates to an electronic device using the above mentioned display as its display.
2. Description of the Related Art
Recently, flat displays are used in many fields and places, and the importance is growing with the progressing of computerization. Nowadays, the typical examples of flat displays are liquid crystal displays (LCD), however, as flat displays based on a different display mode from that of LCD, organic EL, inorganic EL, plasma display panels (PDP), light emitting diode displays (LED), vacuum fluorescent displays (VFD), field emission displays (FED), and the like are being actively developed. These new flat displays are all called a display of self light emitting type, and are significantly different from LCD in the following points and have excellent features not observed in LCD.
LCD is called a light receiving type in which a liquid crystal itself does not emit light and acts as so-called shutter allowing permeation and shutoff of outer light, constituting a display. Therefore, it needs a light source, and in general, a back light is necessary. In contrast, that of light emitting type does not require a separate light source since the apparatus itself emits light. In those of light receiving type such a LCD, a back light is constantly on, irrespective of the form of displaying information, and electric power approximately the same as that under the entire display condition is consumed. In contrast, that of self light emitting type has a theoretical merit that consumption of electric power is smaller as compared with a display of a light receiving type since only portions required to be on depending on display information consume electric power.
Likewise, in LCD, since dark condition is obtained by shading light of a back light source, it is difficult to inhibit light leakage completely, even in small quantity, while in a display of self light emitting type, no light emitting condition is directly dark condition, therefore, theoretical dark condition can be obtained easily, and a display of self light emitting type is overwhelmingly excels also in terms of the contrast.
Since LCD utilizes polarization control by double refraction of liquid crystal, there is so-called strong visibility angle dependency, which display condition varies significantly depending on observing direction, while in the case of a display of self light emitting type, this problem scarcely happens.
Further, since LCD utilizes alignment change derived from the dielectric anisotropy of liquid crystal which is an organic elastic substance, the response time against electric signals is theoretically 1 ms or more. In contrast, in the above-mentioned technologies being developed, so-called carrier transition such as electron/hole, electron discharge, plasma discharge and the like are utilized. Consequently, the response time is in “ns” order, and incomparably faster than that of liquid crystal, causing no problem of remaining of animation derived from slowness of the response of LCD.
Among them, study of organic EL is particularly active. Organic EL is also referred to as OEL (Organic EL) or organic light emitting diode (OLED: Organic Light Emitting Diode).
An OEL element and OLED element have a structure in which a layer (EL layer) containing an organic compound is sandwiched in between a pair of electrodes of an anode and a cathode, and a lamination structure of “anode electrode/hole injection layer/light emitting layer/cathode electrode” such as of Tang etc. is a basic structure (see Japanese Patent No. 1526026).
While a low molecular weight material such as Tangs etc. is used, Nakano et al. use a high molecular weight material (see Japanese Patent Application Laid-Open (JP-A) No. 3-273087).
Further, improvement in efficiency using a hole injection layer or electron injection layer, or control of light emitting color by doping a fluorescent dye and the like to a light emitting layer, are also conducted.
As the method for manufacturing a display using organic EL, formation of a light emitting layer by discharging a light emitting material using an ink jet discharging apparatus is known, and some methods are conventionally suggested (for example, see JP-A No. 2002-164181, International Publication No. 00/59267 pamphlet, JP-A No. 2001-85161 and JP-A No. 2001-341296).
In conventional methods, as described below, an electrode disconnection at a partition corner part which retains light emitting material ink in the form of solution and unevenness of the EL layer thickness due to a meniscus phenomenon are major problems. In JP-A No. 2002-164181, by smoothing the form of a partition as shown in FIG. 4, a problem of disconnection at a partition corner part can be solved. However, a problem of uneven thickness of an EL layer due to a meniscus phenomenon is not solved.
The problem of uneven thickness due to a meniscus phenomenon occurs not only in an EL light emitting layer but also in other functional layers, for example, a hole injection layer, hole transportation layer, electron injection layer and electron transportation layer when formed from a solution.
In manufacturing method for organic EL display by ink jet method, a method of evaporation drying a solvent forcibly, when the light emitting layer in the form of ink solvent is formed into a film, is already reported as a conventional method for solving the problem.
For example, in the International Publication No. 00/59267 pamphlet, a light emitting material which has been made into ink using a solvent having high boiling point is fed and distributed on a substrate, then, the substrate is heat treated. This procedure is performed to obtain an effect of drying a substrate by heating, even after formation of a light emitting layer on the entire surface of a substrate, by using a solvent of high boiling point to slow the evaporation speed of the solvent and to elongate air drying time. However, removing of a solvent of high boiling point completely cannot avoid a problem that heating treatment at higher temperature is necessary, leading to deterioration of a light emitting material. Though deterioration is not observed in the initial light emitting property, this problem exerts a large influence particularly on shortening of light emitting life. If heating treatment is not conducted at sufficient high temperature, a problem of heat deterioration of a light emitting layer will not occur, however, its leads to significant deterioration of the reliability of a light emitting layer due to the remaining of a solvent in a light emitting layer formed as a film.
In the JP-A No. 2001-85161, heating treatment is conducted at higher temperature than the softening point of a material of a light emitting layer, and there is a problem of deterioration as described above.
The method for manufacturing an organic EL display by an ink jet method will be described. As shown in FIG. 5, an EL material ink in the form of solution is precisely discharged to predetermined openings on a substrate by a finely processed nozzle.
Usually, when a light emitting layer is formed by an ink jet method, a light emitting material ink is discharged per one pixel in a dot, as shown in FIG. 6. Partitions are formed according to this. FIGS. 3 and 4 show cross sectional view of an arrow AB or an arrow CD of FIG. 6.
When light emitting layers of the same light emitting color are formed for plural adjacent pixels, such as a data line of passive matrix display, or even for an active matrix display when pixels are arranged in stripes, light emitting layers of the same color can be formed on the data line as well. In such a case, partition openings can also be formed in lines as shown in FIG. 7. In this case, in addition to the ink jet method, formation of a light emitting layer by so-called dispenser method is also possible.
In FIG. 5, the surface of the substrate is drawn as a flat surface. However, actually as shown in FIG. 3, there are partitions having a height of about 5 μm is formed on a substrate to retain the discharged ink. When the solution is discharged to inside of such fine partitions, formation of so-called meniscus surface condition, by the surface tension of liquid, cannot be avoided. When an EL material ink is dried by evaporation of a solvent under this meniscus surface condition, the meniscus surface condition as ink state is reflected as it is, and the thickness of an EL layer becomes uneven as shown in FIG. 3.
When an EL material ink is dried by evaporation of a solvent under this meniscus surface condition, the meniscus surface condition as ink state is reflected as it is, and the thickness of an EL layer becomes uneven as shown in FIG. 3. Conventionally, with such an EL layer having uneven thickness, even light emitting condition within a pixel could not be obtained. Particularly, there was a problem that gradation displaying control, which is a half tone, is difficult.
Otherwise, a problem of disconnection of facing electrodes is also important. Usually, since a facing electrode is formed by vapor depositing a metal thin film, thickness from 100 nm to at most 500 nm is a limitation for stable formation. When thicker than this, a risk of peeling increases due to the tension of a metal itself since it is no longer a thin film. With thickness in this range, when a partition has a height of 5 μm or more, disconnection tends to occur at a corner part of a partition 200, as shown in FIG. 3, and a lot of defective pixels occur in which electric field is not applied to an EL layer.
In the above, a case where an EL layer is formed by an ink jet method is mainly explained. That is because the problem of uneven thickness is most likely to occur remarkably when ink jet method is used. However, in cases where an EL layer is formed by other methods, it is not easy to completely solve the problem of uneven thickness within a pixel. For example, in casting method, layer-by-layer self-assembling method, spin coating method, dipping method, and the like, the problem of uneven thickness as same as an ink jet method will likely to occur since they are wet processes. Further, in vapor deposition method, cases where partitions as the above are used in order to keep a mask and a substrate surface from being in contact. And in that case, the thickness may become uneven near the partitions, leading to the same problem in such a case.